[evlatests] A short summary of 3-bit results

[Barry Clark]

Item 4 - the fringe rotator does not have unit gain, but a gain of
4*cos(pi/8)/pi = 1.176.

And autocorr doesn't change when you turn off the lobe rotators
because they are not turned on for autocorr.

Rick Perley wrote:
>     After a couple of hectic weeks of testing, I thought a short summary 
> might be useful:
> 
>     1) Noise measurements of the astronomical signal when observing 
> blank sky show the 3-bit samplers give 20% higher noise than the 8-bit 
> samplers.  This is a very stable and repeatable result.  At most, a 5% 
> degradation is expected. 
>     2) The 8-bit samplers noise values are at the theoretical level 
> (~0.13 Jy rms for 2 MHz spectral channel width and 1 second averaging) 
> for the accepted SEFDs at C-band (the band we have done all testing at). 
>     3) Both polarizations give the same results, although there is some 
> small chance the RCP is slightly higher (previous observations have been 
> skewed by two of the samplers on that side misbehaving). 
>     4) Turning off fringe tracking causes the noise values to drop by 
> 20%.  This happens for *both* 3-bit and 8-bit data paths, providing 
> strong evidence that this effect is a gain change, not an improvement in 
> sensitivity (since the 8-bit data are believed to be at theoretical 
> values with fringe tracking on).  But if this effect is a gain change, 
> why do the autocorrelation amplitude remain unchanged when the fringe 
> tracking is turned off? 
>     5) Comparison of the cross-polarization antenna autocorrelations to 
> the same-polarization antenna autocorrelations, when the same signal is 
> provided to both samplers shows the cross powers are about 92% of the 
> auto powers. 
>     6) All other aspects of 3-bit testing have given good results so 
> far:  Bandpass shapes, phase and amplitude stabilities all look good.  
> There are some concerns about closure, but more testing of this will be 
> needed before definitive statements can be made. 
> 
> 
> 
>    
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